My copending patent application Ser. No. 698,043 which was filed Feb. 4, 1985, is titled POWER TRANSISTOR EMITTER BALLASTING and is assigned to the assignee of the present invention. This application teaches the use of a plurality of individual small emitters connected effectively in parallel through individual ballasting resistors to create a power transistor and is incorporated herein by reference. This application also teaches a distributed sense emitter associated with the power device emitters for responding to hot spots in the power transistor.
U.S. Pat. No. 4,146,903 issued to Robert C. Dobkin on Mar. 27, 1979, and is assigned to the assignee of the present invention. Its teaching is also incorporated herein by reference. This patent teaches the incorporation of a sense emitter in a power transistor in close proximity with the power emitter. Thermal gradients are sensed by comparing the potential developed between the sense emitter and a remotely located emitter. Means are included for turning the power transistor off when the gradient exceeds some predetermined threshold value. This is done by connecting a differential amplifier having a fixed offset potential between the sense emitter and the remote emitter. The differential amplifier output is coupled to the base of the power transistor. Thus, when the potential difference exceeds the offset, the power transistor base will be pulled low so as to shut it off. This arrangement has been used successfully in the LM138 series voltage regulators and has several advantages over prior art approaches. However, it requires a remote emitter that is not heated by the power emitter, or at least is heated to a lesser degree, to produce a gradient response.